U.S. Pat. No. 6,411,195 describes a device that is configured for delivering a code in the form of a series of knocks. The device of this patent has an impeller head that is capable of reciprocation to thereby transmit an encoded series of discrete mechanical impacts to a surface of an impact transmissive body. An impact sensitive transducer is configured for picking-up the vibrations and decoding the data therefrom. A particular example of this device is for access control.
U.S. Pat. No. 6,848,314 describes a device for receiving data transmitted as impulses through an input transmission body. The device has a vibration sensitive transducer which constitutes part of a floating mass assembly with a pick-up portion. The pick-up portion is biased towards a surface of a rigid body that is either the input transmission body or a vibrations transmissive member. The assembly has a freedom of movement permitting it to vibrate independently from the rigid body.